PROJECT SUMMARY Studies of human and mouse prostate cancer indicate that the homeodomain-containing transcription factor Nkx3.1 is an important haplo-insufficient tumor suppressor gene involved in prostate tumor initiation. Nkx3.1 protein expression is lost in human and mouse prostate tumors, and a recent study has found Nkx3.1 mutations in hereditary prostate cancer. We have generated and characterized conventional and conditional Nkx3.1 mutant mice. These mice develop prostatic epithelial hyperplasia and dysplasia and have been extensively studied as a model of prostatic intraepithelial neoplasia (PIN). In these animals, the exit of differentiating prostate luminal epithelial cells from the cell cycle is delayed, resulting in prostatic epithelial hyperplasia and subsequently PIN. Further analysis of Nkx3.1 mutant mice has revealed several features that begin to shed light on the mechanisms of tumor suppression by this protein. Nkx3.1 regulates a class of genes in a dosage-sensitive and stochastic manner, a phenomenon that may underlie haplo-insufficiency and which appears to be dictated by the differential chromatin states of target genes. Nkx3.1 regulates the expression of cell cycle regulators as well as a unique class of androgen target genes. Further, loss of Nkx3.1 dysregulates the expression of pro- and anti-oxidant enzymes (including peroxiredoxin 6, glutathione peroxidase 3 and sulfhydryl oxidase 6) resulting in loss of protection against oxidative damage. It is our hypothesis that loss of Nkx3.1 promotes prostate tumor initiation by deregulating multiple gene programs that alter cell cycle exit, androgen signaling, and the anti-oxidant response. We have outlined specific experi- ments to test this hypothesis according to the following aims: 1) To examine the regulation and function of dosage-sensitive Nkx3.1 target genes. 2) To define the regulation and function of genes induced by androgen specifically in Nkx3.1-deficient cells. 3) To test the notion that disruption of the anti-oxidant defense system in Nkx3.1 mutant mice leads to the accumulation of genetic mutations. PROJECT NARRATIVE The significance of these studies lies in their potential to increase our understanding of the mechanisms of prostate tumor initiation and to generate molecular targets for diagnosis and prevention in a preclinical model.